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Are runways "one-way"?
Do runways always handle traffic in the same direction? Another question asked how runways were labelled left and right (8L and 8R, for example). But of course "left" and "right" depend on your perspective. If you were approaching from the opposite direction, then left would be right and vice versa.
Does this mean that runways only handle traffic coming from the same direction all of the time? Are there no ambidextrous runways that flip depending on the prevailing winds or whims of ATC?

Most runways do flip direction with the wind. The will be ATC's call at a towered field or the pilots' call at an untowered field.

L and R are swapped when looked from the other way, Runways are usually both ways, unless using them the other way is dangerous (e.g. due to a mountain), or not allowed (e.g. due to noise). You'll find a lot of information in the two questions identified as duplicates.

As noted in the linked question to the question you linked to (this one), runways are almost always bidirectional. In your example, the other runway would be simply numbered 18 higher or lower, for 180 degrees off. Thus, the runways would be 26R and 26L - the "L" and "R" would swap, because you are pointed at them the opposite way. You can see this in the diagram of LAX below (src):

Yes, generally they flip based on prevailing winds, though I can't imagine they flip based on the "whims" of ATC often. Most airports will have a "calm wind runway", which is the preferred runway for operations in calm or near-calm winds.
Due to terrain or similar factors, some runways may be only usable one direction, but both directions will still be numbered, as you will takeoff and land pointing different directions (for example, Lukla airport in Nepal). Alternatively, as Jan Hudec noted, some will operate one-way, such as TNCM, though again, both runways are numbered. Occasionally, the opposite direction is not numbered, though this is rare.

Actually, there are runways which have landing procedure only in one direction due to terrain, but do permit take-off in the same direction as landing, making them effectively one-way since using the same direction for both is safer. Example is TNCM (Pricess Juliana International Airport).


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How would a detachable cabin module work?
Ars Technica recently reported that Airbus filed a patent for a detachable cabin module that would speed loading and unloading of passengers (otherwise known as self-loading freight).

This does seem like it might improve turnaround time since passengers are rather slow to self-load and self-unload. That's not my question (which is in fact an earlier question).
My question is that I had always imagined the strength of an airframe was due to its long cylindrical shape. This is both in terms of it as a pressurized tube as well as lateral strength from bending forces. The detachable cabin module would seem to defeat all of these advantages. Am I correct in this understanding? Are there ways that the Airbus patent mitigates them?

You might find the information you want in this question: https://aviation.stackexchange.com/questions/21523/why-is-the-fuselage-on-an-airliner-circular-shaped?rq=1

Possible duplicate of Why don't airlines load their passengers like freight?

I've retracted my close vote since I don't belief that the proposed dupe answers the OPs key question - The detachable cabin module would seem to defeat all of these advantages. Am I correct in this understanding? Are there ways that the Airbus patent mitigates them?

Patents are often used to prevent some vague principle to be used by competitors, this doesn't necessarily indicate this direction is currently explored. I doubt this is the best way to decrease boarding time. Is there a need to speed up boarding to start with? The limit in takeoff and landing rate seems to be the problem nowadays.

If they made the basic fuselage the same way a C5 is, with an opening nose, then it would be (relativly) simple to open the nose, slide in a pre-loaded, unpressurized passenger/cargo tube which automatically latches itself to the rear bulk-head, then close the nose, which then seals itself and allows the craft to be pressurized. Beyond that, I don't see much value in the Airbus concept.

You are right, this patent looks like it will not work in reality.
Fuselages have to carry two primary loads:

Pressurisation loads, which cause uniform tension in lengthwise and circumferential direction, and
bending loads from inertial and control surface loads, which create both tension and compression loads in lengthwise direction, plus some shear.

Reducing the height of the fuselage will drive up the strain from the bending loads, so more material is needed to transmit them. Also, the bottom of the passenger compartment and especially the corners at the lower edges will experience much higher loads than a cylindrical fuselage ever would under pressurization. Again, to make this work needs a much beefier structure.
The loads could be kept the same if the passenger compartment is tightly bolted to the remaining aircraft structure such that the combination of "flatbed" fuselage and passenger compartment can be considered as a contiguous structure, but this would need redundancy in the connections and would make fitting the passenger compartment non-trivial. Airbus is arguing in this direction, but I wonder how much time can be saved and how much maintenance time is added if the primary structure of the fuselage is torn apart and re-joined at every stop. Note that you not only need to connect the parts at each end, but to transfer pressurization loads you need to bolt them together along the full length of the passenger cabin as well.
Today's airliners are already designed to survive partial depressurization, such that the lower fuselage is depressurized without the pressure loads in the upper part causing damage. Therefore, the fact that both parts can be split and can depressurize independently of each other does not add additional load cases. However, the structure today is dimensioned to sustain those loads only for a short time. Pressurizing only the passenger compartment or not joining the parts along the full length for every flight will need a much stronger structure than what is used today, so the weight impact of a simpler way of joining both parts will most likely result in an economically unattractive outcome.
I did not go into detailed calculations, but I strongly suspect that this idea will not be adopted by airlines. Besides the structural impact, such a way of loading passengers would complicate logistics, since a fitting passenger container plus facilities for mounting it on the aircraft must be available at the departure and destination airfields.
Lastly, proposals like this are not exactly new, which is why the first claim extends over 73 lines to be narrow enough to not conflict with earlier patents or publications. If there had been some value in this idea, we would had seen it being adopted long ago.
In helicopters, however, which fly at altitudes where no pressurization of the passenger compartment is needed, this idea has been tried out before. See below a picture of the Sikorsky S-64 Skycrane carrying a container (picture source) in which passengers could be carried.

Yeah, my immediate thought was "how can they patent this? Sikorsky already did it!"

@egid: Hard to see how Sikorsky could've patented it, given that Fairchild came up with the idea years before they did (and with a fixed-wing airplane, at that!).

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Could I build an airship (rigid dirigible) that used a vacuum rather than lifting gas?
There are other questions (c.f. What gas is used in airships to give them buoyancy?) that discuss the use of lifting gasses such as Helium and Hydrogen in airships. Helium is rare and Hydrogen is flammable.
If one could build a light-enough and strong-enough rigid outer frame, then couldn't one use a vacuum rather than lifting gasses?  At ground level, the frame would only have to withstand 15psi (100,000 pascals), which isn't a tremendous amount of force. Theoretically, a vacuum should provide even greater lifting force than any gas.
Have there been any attempts at vacuum based rigid-dirigibles -- and if not, what are the technical or mechanical limitations to doing so?

http://en.wikipedia.org/wiki/Vacuum_airship suggests that you would require a material with a lower propensity to buckle than pure diamond.

@Hugh Dropping the assumption that the shell is a homogeneous material may allow lighter and stiffer structures [than diamond]

which isn't a tremendous amount of force.?  It's an enormous force.  About one ton per square foot if you are on the old system still.

I'm thinking graphene here, but it's a long way away.

"Which isn't a tremendous amount of force." Actually, it's pretty big.

who says it has to be a single large compartment that is evacuated of air?  why not lots of small, lightweight (carbon fiber?) compartments.

@Michael it looks like a vacuum is just 179 grammes per square meter lighter than helium, if I understood pericynthion correctly. I'd be surprised if it's possible to make such compartments without using more than 179 grammes of material per square meter

Lighter than air craft work by displacing air from an enclosed volume with a gas which has less mass per volume (ie less density) than air. The achieved lift per volume = mass_new gas - mass_air - mass_enclosing structure. When vacuum is used the lift = mass of air per volume less support structure. As air masses about 1.2 kg/m^3 at STP (~= sea level, 25C)  the support structure needs to mass less than 1.2 kg of enclosed volume to get any lift at all.  Larger volumes gain some advantage due to cubed-squared law as volume increases with side cubed but area wity side squared. HOWEVER ....

.... the force involved get "out of hand" rather rapidly at any scale (as others have noted with references to calculations elsewhere.

The answer of Akhmeteli deserves more attention: It can be done and attempts are being made to patent the solution.

It can be done with nonexistent materials, but there's thoroughly no reason to do it.

what are the technical or mechanical limitations to doing so?

There is no known material that has a sufficient combination of lightness and strength to support a vacuum that would support the weight of the container on Earth at sea-level.
There was a similar question with a good answer on Physics.se

Beryllium Vacuum Sphere Boat/Aircraft

Is it possible to make a solid rigid evacuated "balloon" out of Beryllium or other elements or alloys?

The currently first answer runs through the maths

calculated it over a database of almost 3700 materials including all common aerospace composites, metals and alloys.

and concludes

So in short, the best materials fall short of achieving our aim by a factor of just over 2. It is conceivable we might push up the stiffness / density ratio by perhaps developing some kind of Beryllium foam. For example, a closed cell foam of this Beryllium with a relative density of 0,041 would give a value of about 920 at the expense of lowering the Young's modulus to about 600MPa - however, I have no idea if such a foam is even possible. Alternatively it might be possible to come up with some clever engineering of the envelope geometry to overcome the buckling constraint. However, I suspect the effort is unlikely to pay off with a better boyancy ratio than is already achievable by conventional balloons.

Other related questions

Is it possible to create a vacuum ballon using concentric partially pressured spheres?

What if you only wanted to add some bouyancy to an otherwise 'normal' airplane or something? We're making a huge assumption, that we want to float the whole ship by itself. What if we just want something that's "light" enough to be a better ground effect vehicle or something like that? Where's the cutoff - how much bouyancy could be acheived?

@Jasmine: It's not a case of cutoff, you can not achieve any buoyancy at all with this approach, you can only make your vehicle heavier. The vacuum balloon cannot lift itself, let alone anything else. Even if you constructed your vehicle from metal foam and somehow evacuated the gas (air) from the foam, it would almost certainly still be either heavier than conventional construction or too weak to support the structural forces on top of atmospheric pressure.

Oh thanks, that makes total sense.

@Jasmine: Even if one had a magical magical vacuum foam which weighed nothing and could be easily controlled, such a thing would be of very little use in an airplane.  Replacing a cubic meter of interior airspace with magical vacuum foam would add less than 1.5 grams of lift.  The interior volume of an Airbus 380 is less than 4,000 cubic meters, so replacing all of it with the magical foam couldn't generate more than 6 kilograms of supplemental lift.  The only way to add a meaningful amount of lift would be to make the craft a lot bigger, and the only way that would save any fuel...

...would be if the craft's airspeed was extremely low.  Having an airship stay aloft at zero airspeed requires almost no energy, but propelling it any significant speed will require a lot more energy than would be necessary for an airplane of comparable payload capacity.

The other answers correctly point out the extreme (impossible) materials strength and stiffness requirements of a vacuum balloon.  Another notable point is that the advantage in lifting capacity is small; at standard temperature and pressure the buoyancies of helium, hydrogen and vacuum are respectively 1.096, 1.185 and 1.275 kg/m^3.  So even if you could hypothetically build a vacuum balloon with the same envelope mass as a hydrogen or helium balloon, it wouldn't be able to lift much more payload.

Beneath the argument that it's impossible to build a structure which can withstand the outer pressure, pericynthion pointed to another argument:
The buoyancies of gas and vaccuum is similar and the higher mass of a vacuum-airship will cancel the benefit.
But there is even more:
Airships usually include a balloon filled with air. If the ship ascends, the gas expands and the air is pressed out. (see http://www.americanblimp.com/fly.htm)
This way, the inner and outer pressure is equal and there is no additional stress on the hull due to overpressure.
Also, the buoyancy does not decrease: In an altitude of about 5.8km, air pressure is reduced to 50%, and so is the buoyancy for a fixed volume. If you let the gas expand, it will expand to twice its volume. Twice the volume times 50% buoyancy gives you 100% buoyancy again.
But a fixed volume of vacuum will not expand and have only 50% of its buoyancy at sea level.
Of course, 5.8km altitude is very much for an airship, but the numbers are nice to show the effect. And due to the exponential nature of the air pressure, a gas-filled airship which allows the gas to expand has a higher buoyancy than a vacuum airship with fixed volume above 600m for hydrogen filled and 1200m for helium filled ships. Here is a plot comparing the buoyancy of 1m³ of vacuum to 1m³ gas at sealevel:

(By the way: This is the reason why weather balloons have a flexible hull or seem to be partially filled only at sea level)

All of the links I've seen are to sites where it is implicitly or explicitly assumed that the dirigible is a homogeneous sphere. This approach does have some merits - any failure has to originate at some point, and if the sphere is absolutely uniform then there is no obvious point of first failure; and in the analogous situation of deep sea exploration spherical construction has been shown to have its uses. However, in practice no material construction is ever completely uniform, and even if one was, until the dirigible was aloft there would be a difference between those points which were in contact with the ground or supporting structures, and those which were not.
It seems to me that a much better approach would be to have something which is approximately spherical, but which is supported internally by a complex system of ribs and buttresses, akin to what can be found in a gothic cathedral.
For a starting point, take a look at e.g. http://www.cutoutfoldup.com/905-spherical-model---cube.php.
Imagine this structure scaled up and manufactured to extremely high precision using the strongest available materials, and covered by a thin film so that the interior is completely enclosed. What you'd have is essentially a sphere which is very thick at certain points (i.e where the ribs are) but very thin at others. The ribs would be able to withstand enormous pressures. The other points - not so much. As it stands, it would fail at one of the non-ribbed points. So, not a great advance. But now imagine that the material of the faces is quite strongly curved; that each face is a portion of a sphere which is much smaller and therefore more strongly curved than the overall sphere. These smaller spherical segments should be able to withstand a much larger external pressure than a large sphere of the same thickness.
Now imagine each of these bubbles is itself ribbed, with the facets between the ribs occupied by still smaller ribs and bubbles. The construction is effectively fractal, with the repetition at smaller scales occurring as often as is required. The very smallest bubbles would be very highly curved, and therefore very strong. The external pressure exerted on them would be directed to small ribs, which would in turn be directed to larger ribs, and so on. I don't know of any attempts that have been made to construct a vacuum dirigible along those lines, but I don't see why it shouldn't work.
EDIT: If we understand that any vacuum achievable on Earth using a mechanical pump is in fact just a partial vacuum, and we therefore use the word "vacuum" as an abbreviation for "partial vacuum", it follows that "vacuum dirigible" and "lifting gas dirigible" are not mutually exclusive terms. It should be possible to construct a dirigible containing hydrogen or helium at significantly reduced pressure, instead of the slight overpressure that is normal. The rigidity would then have to come from the structural rigidity of the shell instead of from the internal pressure.
The big advantage, in the case of a reduced pressure helium dirigible, is that under normal circumstances the loss of helium to the atmosphere should be negligible, unlike a normal balloon where losses are substantial and have to be continuously replenished. The major gas seepages would be from the higher pressure exterior to the lower pressure interior, and any potentially escaping helium would be in effect swimming against that current. It would also be seeping through a rigid material which is likely to be more impervious to the flow than a more flexible material would be. This advantage will become increasingly significant as the price of increasingly rare helium continues to skyrocket.
Not that it should be necessary to employ reduced-pressure helium, it's just that doing so would make construction easier by decreasing the pressure difference between interior and exterior to something more easily manageable. But I'm fairly sure that with an intelligently designed shell it would be possible to create a working dirigible even with the interior close to being a hard vacuum. The article cited by RedGrittyBrick in his answer states "the best materials fall short of achieving our aim by a factor of just over 2", and I am sure that a better structural design would result in MUCH more than double the rigidity of a simple homogeneous sphere for the same mass of material. Supercat's anecdote, below, helps to illustrate that.

When my father (and lots of helpers) built a sandbag wall around his house (which held up to a flood with standing water almost 4' above ground level, plus substantial waves on top of that), rather than building it as a circle or ellipse, he built it as a scallop-shape, with substantial buttressing at every concave point.  That greatly reduced the radius of, and consequently increased the strength of, all of the non-buttressed portions of the wall.

@supercat. That shape also deflects cannon fire.

the reduction in helium leakage seems compelling.  wonder what the cost advantage would be...

According to our finite element analysis (US patent application 11/517915 (Akhmeteli, Gavrilin, Layered Shell Vacuum Balloons, you can find it at USPTO site or at http://akhmeteli.org/wp-content/uploads/2011/08/vacuum_balloons_cip.pdf ), it is possible to construct a vacuum balloon using commercially available materials and sandwich structures. In his comment, @Hugh refers to a Wikipedia article that emphasizes our calculation proving that one cannot make a vacuum balloon as a homogeneous spherical shell made of currently available materials, but does not clearly quote our conclusion on a possibility of inhomogeneous (sandwich structure) spherical shell vacuum balloon.

Sandwich material, of course. Like stated in a comment of the physics site, this is awesome.

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How does the Eurocopter Blue Edge and Blue Pulse rotor blade design work?
In response to my question about quieter helicopters,  KeithS mentioned the Eurocopter Blue Edge helicopter blade design.
According to an article I found on them, there appears to be two components at the very least: a boomerang style rotor tip (Blue Edge) and piezoelectric active flaps (Blue Pulse) on the trailing edges.
How do these (as well as other changes) affect the noise level (i.e., what is the aeronautic principle behind their design)? Also, the sound reduction itself seems relatively modest (3-4 decibels), but is the quality or tone of the sound perceptually different such that it seems much quieter?

EC155 helicopter. Compared with a conventional blade, the Blue Edge has a tip with both forward and rearward sweep.
This gives the blade more time to interact with the vortex 'shed' by the rotor, reducing the characteristic impulsive (slap-slap) blade-vortex interaction noise generated by a helicopter.
The active trailing edge flap harmonises the frequency thus making it quieter.


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DA40 fuselage static ports / G1000 not in AFM
I’m a flight instructor who just switched from instructing in 172’s to flying the Diamond DA40 and am confused about a few things. Our DA40 has two static ports, one on either side of the fuselage. This doesn’t make sense to me because the AFM in the airplane says the static ports are on the left wing along with the pitot probe. I haven’t been able to find anything online about these static ports, and I’ve only seen one DA40 AFM.
My second confusion is, the airplane is equipped with a G1000; however, the AFM in the airplane doesn’t say anything about the G1000, there is only a cockpit reference guide for it. Would this indicate an upgrade from original avionics? Would it be in the maintenance records? (Mx records was my next stop)

I found the answer by further reading the AFM. On DA40's with an auto pilot installed, optional static ports may be added to the fuselage.

My guess if you have an older airplane with a separate autopilot that is not integrated with the G1000.
If you have an autopilot with a separate static system, such as the KAP-140, there will be additional static ports. They should be noted in the AFM, but in the supplement section referring to the KAP-140 autopilot.
In contrast an airplane with a G1000 integrated autopilot (GFC-700) the autopilot will share airdata with the G1000, so it will not have an extra static port, but rather just a wire to get airdata from the GIA.

I'm doing my flight training on DA40NG. Are you thinking about alternate static valve? It's correct that there is static port on left and right side of fusalge more like rear and pitot tube on left wing, also stall warning port on left wing leading edge. About G1000 shouldn't explain alot on AFM but you should read more on G1000 manual. I used to watch a video on youtube an hour long explained about G1000 all techincal stuff and more.

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Is there any requirement that airports have distinct names?
Is there any requirement that airports have reasonably distinct names from each other?
I've known about SYD versus YQY for a while, but I've just read that there's two airports mentioning Genghis Khan: ULN in Mongolia and NZL in Inner Mongolia in the PRC.

No. As well as your examples there are two airports called Billy Bishop in Ontario.

That's why names are not used for flight planning =)

@BurhanKhalid the pilot in this article must have used names: Airport mix-up sees Winter Olympics delegation land in Pyongyang after pilot files wrong flight plan

There's no requirement for names. Location codes can't be the same though.

Rather than a location code, I believe the correct term would be the airport identifier.


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Why didn't government regulators prevent MH17 from flying the route it took?
Why wasn't MH17 prevented by government regulators from flying the route it took?
MH17 wasn't the first case of governments or combatants downing commercial aircraft: Iran Air Flight 655 and Korean Air Lines Flight 007 come to mind.
I find it hard to believe that if an area had an unacceptably high risk for aircraft, that it would be up to the individual airlines to decide not to fly that route, rather than governments being flown over, or the source or destination countries, flat-out forbidding it unless there's no alternative.
Was military aircraft (such as Ukrainian Air Force Ilyushin Il-76 and other incidents mentioned in the Background section of Wikipedia's article on MH17) being shot down in the area a new phenomenon, with regulators not acting fast enough to deal with this new information?

I think your question would benefit from a rephrasing. The title question is quite broad (and not really well suited for the site as currently phrased), while the final question you have in your body could be made to fit well, but needs polishing (which information, what would have been "fast enough"?).

@Federico does it look better now? Or do you still want "fast enough" clarified?

It should be noted that it is still not clear, who was the target.. Other than MH 17, SQ 351 and AI 113 were flying within 25 km of MH 17..

The Airlines can decide for them selves which route to take, however the government has an intelligence operation which can predict events that might happen in political unstable regions of the world. However in the case of MH17 it was not the "army" that shot down a plane but an conflict between rebels and the government.
Routes flown to Kuala Lumpur
While the flight from Amsterdam to Kuala Lumpur from Malaysia airlines have been suspended, one can still retrieve the route flown via flightaware.com:
The KLM flightpath (KL809) 10246km:

The Malaysia Airlines flightpath (MH19) 10855km:

From the plots it can be seen that both Airlines diverted their routes after the accident with MH17. They both avoid Ukrainian airspace.
Would flying these routes prevented the accident? Yes, but at the time intelligence agencies where not aware that the Separatists had acces to such advanced surface-to-air missiles. This also because the nature of the conflict would not suggest that such weapons would be required.

could you please consider adding more details to your answer? also, how would those plots be relevant?

Had the downed flight taken either of those paths there would have been no tragedy

I really, really like your answer including intelligence discussion and an argument about this kind of weapons not being expected or predicted in this area due to nature of conflict.

@TomMcW, that is easily said afterwards, during that time the separatist were not seen as a threat to commercial aviation at cruising altitude. Hence we still fly over Iraq.


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Why does the cabin pressurisation switch have a manual mode?
Reading about Helios Flight 552 it is clear that the pressurization switch on a B737 has two modes: AUTOMATIC and MANUAL
Since it appears that one of the causes of the accident was an incorrect setting of the switch to MANUAL and due to hypoxia the first officer didn't spot the problem, why is it that a MANUAL setting is offered if it can lead to such disasters?
What is the use of the MANUAL position of the switch if the setting has to be set to AUTO for the flight?

What if the automatic control fails?

If you follow that idea further, then you soon end up asking why not automate everything?. Also see this and this about pilots' control over various systems.

True. But what I am asking is why a manual and AUTO. Why is it so easy to switch to manual if consequences can be disastrous? Not arguing on the fact that everything has to be automated.

I believe the point of the question really is: at what point during taxi or flight would Manual be selected instead of Auto? It's obviously there for a reason, what it that reason?

There was no hypoxia when the mechanic and co-pilot failed three on-the-ground opportunities to detect the incorrect switch setting...

The question is incorrect -- there is no OFF mode.

The B737 Next Generation models (737-600, -700, -800, and -900) all use a digital cabin pressure controller, which has AUTO, ALTN, and Manual modes.  Older 737's may use this digital controller or an analog controller with Auto, Standby, Manual AC and Manual DC modes.

From MyAirlineFlight.com, which also has a depiction of the older, analog panel.
In the digital panel, the Auto mode selects between two digital controllers which swap every flight so that if #1 is primary (in control in the Auto mode) this flight, #2 will be primary next flight.  The ALTN mode makes the "other" controller primary, so that if the primary controller fails, the other one can be selected.  With both the digital and the analog controllers, the Manual mode gives the pilots direct control over the outflow valve.
There are several reasons that this control is sometimes desirable; none of them are particularly common.  First, if you need to perform a ground pressurization check, the pilot or mechanic uses the manual mode to close the outflow valve and allow the aircraft to pressurize on the ground.  It isn't common, but there are some checks that require it.  Second, the manual mode is used to verify that the outflow valve DOES open and close.  This used to be a check performed by the pilots, but (in large part due to the Helios accident mentioned), it is now a maintenance-only function for many operators.
Third, if the flight must be conducted unpressurized, the manual mode allows this -- open the outflow valve fully, and there is no pressure differential -- say if you had a damaged window that needs to not be pressurized while the aircraft is ferried to where the window can be replaced.
Fourth, in the case of an emergency evacuation, it may be necessary to quickly depressurize the aircraft, and the manual mode allows the crew to open the outflow valve with only DC (i.e. only battery) power available.  (The automatic modes need AC power to operate.)
Finally, if both digital controllers fail in flight, then the crew needs a way to control the pressurization, and the manual mode is used for this.  Getting to this point is quite rare, but it can happen.  Controlling the pressurization manually takes a lot of time & attention by the First Officer, so it isn't something that's practiced in the aircraft, and because the really big cues that you're doing it well or poorly come from what your ears feel, it isn't something that's done in the simulator either.  But there has to be a way to manually control the pressurization, so the Manual mode is it.  (Loss of all generators in flight could also put you in this position -- with no AC power the Automatic mode is unavailable.  Again, an extremely rare event.)
With the removal of the outflow valve check from the pilots' preflight, the switch is typically left in the AUTO position always, unless maintenance is doing their checks.  More than a few times, crews got distracted in the middle of the outflow valve check, left the switch in manual, and never got back to it.  The usual result was a Cabin Altitude Warning shortly after climbing above 10,000'; if the crew recognized their error, they could simply switch back to Auto and continue.  If they didn't realize that they'd done, they'd shortly get a call from the back asking why the masks had dropped, and that was usually the big clue that things had gone wrong & they needed to go back & land.
One might wonder why the Manual mode is annunciated by a green light rather than an amber one -- the amber light would show up when checking the Master Caution.  That was a design decision by Boeing a few decades ago, and I don't have insight into their thinking there.  (Much like, why did they use the same horn for Takeoff Configuration Warning as for Cabin Altitude Warning... it evidently seemed like a reasonable idea at the time.)

Also in case of ditching, it may be desirable to close the relief valves to extend buoyancy time.

Wouldn't AC power be available even when on battery, via the emergency inverters?

@Sean The inverter provides some AC power, but the load to move the outflow valve may be more than it can deliver. Going back to the analog pressurization panel, the "Stby-AC" mode moved the valve rapidly, while the Stby-DC mode was much slower. My inference there is that the AC motor used more power, since AC power is typically plentiful, while the DC motor used less since its envisioned use case was to run from the battery.

Regarding unpressurised ferry flights, why not simply pull the pressurisation system's circuit breakers?

@Sean Because you have the switch on the pressurization control panel. Pulling circuit breakers to do what you have switches for, has gotten crews in serious trouble, because it's far too easy to lose track of what systems you no loner have working. And the interrelationships between them.


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Can an aircraft at 10,000 feet have sea level pressure in the cabin?
Right now at my university we are busy with a project for designing a conceptual aircraft. Our aircraft is in the CS-23 range, this airplane is a small cargo aircraft with the option for carrying injured passengers (so like an ambulance aircraft).
Our aircraft will have a cruise height at 10,000 ft. We want to make this aircraft pressurised but our question is therefore, is it possible to pressurise the aircraft from inside below the normal 8,000ft. So for example, we are flying at 10,000 ft but we want to have the inside pressure having the same as on sea level if that is possible, or is it only possible to pressurise the inside at 8000ft.
Thank you in advance.

Hello Guido, welcome to aviation.stackexchange.com.

Hi Guido, welcome to SA, you may be interested in reading this, at the end of the day, you may not need a pressurization system at all!

@GHB, aircraft cruising at 10,000 does not need either pressurization nor supplemental oxygen. 10,000 ft is considered safe. Rare medical conditions where it might be a problem are more easily handled by portable oxygen; such patients are likely to need a ventilator, at least available, anyway.

If it's airtight, the pressure will remain at whatever it was when you closed the door before take-off.  Whether your fuselage can withstand the pressure difference when it gets to 10'000 feet is entirely down to you.  Aircaft are generally pressurised to around 8'000 feet because the fuselage is not storng enough, to keep down weight, and people can adjust easily to that pressure altitude.

@Simon:  If the cabin is hermetically sealed to maintain the pressure when the door was closed, two very bad things will happen:  You'll run out of oxygen in a matter of hours, and the humidity from exhaled breaths will fog up all the windows and begin to condense on all the instruments.    Pressurized airplanes continually bring in fresh air and cycle out the air already in the cabin.  The airplane is not literally sealed tight.  It just has a compressor (and heater and humidifier) between the outside air and the cabin.

@abelenky I know.  I was simply saying that it is entirely down to the designer what happens using all the usual compromises.  There is nothing preventing someone from designing an hermetically sealed fuselage pressurised to sea level.

8,000' is not a "Normal" cabin pressurization altitude.  It is the MAX allowable cabin altitude for a pressurized aircraft.  Take any pressurized aircraft and fly it below it's max cruising altitude and the cabin alt will be far below 8,000'.  Flying at 10,000 feet you will be able to maintain the cabin much lower than 8,000' with even a very modest cabin pressure differential. One exception to this is when flying to Bogota at 8,361'.  Airline SOP requires you to keep the cabin at 8,361' for the entire flight. I would always keep the cabin at 6,000' and then raise the cabin prior to landing.

@JanHudec If you are transporting someone who was injured or became ill while scuba diving, and they are not currently suffering decompression sickness, you might be able to transport them safely without using a decompression chamber if the cabin is pressurized to sea level, but you might not want to expose them to any lower pressure. Oxygen is not the problem here, it is purely and simply (as far as I understand it) a matter of the ambient partial pressure of nitrogen.

@DavidK, true. I should have remembered that case. It is even advised not to fly for a day after diving.

If you design the aircraft, shouldn't you be answering that question? If it is part of your aircraft's mission to fly at 10.000ft whilst maintaining sea level cabin pressure, then you should design it that way.
Why would it only be possible to pressurize the inside to 8000ft? Not to 9000ft, 7000 ft or any other arbitrary altitude?
Of course the construction of the aircraft needs to be able to withstand the forces caused by pressure difference between the inside of the cabin and the outside. And the pressurization system needs to be able to maintain the pressure at sea level, so compensating for any air leaks in the pressure vessel.
For an initial feasibility study you could look at the pressure difference on the hull when an aircraft is flying at FL430 whilst maintaining cabin pressure at 8.000ft. Note thate the Boeing 787 maintains the cabin altitude at 6.000ft, while operating at 43.000 ft.
Compare that to the pressure difference between 10.000ft and sea level.
Looking at those pressure differences should give you a feeling whether or not it is easy to achieve your objective.

Yes, this.  Specifically the first paragraph.

Pressurized aircraft such as LearJets are designed to maintain sea level pressure up to a certain altitude, say 28,000 feet.  Above that altitude, the cabin pressure begins to fall so that at 43,000 feet in this example, the cabin pressure is about 5,000 feet of altitude.  However, air ambulances can tell the tower what altitude they will fly so that they can maintain sea level pressure in the cabin.
Airambulance.com

Not just medevac, but any aircraft's pilot can tell ATC (not just tower) what altitude they can accept—ATC will provide separation at any altitude. If vertical separation is not available they can vector aircraft around each other. Just because an aircraft has a specific service ceiling does not mean they have to fly that high 100% of the time.


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What prevents a propeller from detaching completely from the engine body?
How does a propeller remain attached to the engine? I know there is a gearbox and/or some other mechanism that transmit the movement but is there any security mechanism that avoids it from completely detaching from the main engine? There is a huge amount of forces acting on that tiny spot where the propeller is attached to the engine, especially on turboprops, and this is where my question comes from.

Do “sufficiently strong bolts” count as “security mechanism”? I am not really sure what you expect here.

Now consider a helicopter rotor. The forces are even greater, as is the importance of keeping it attached. For that reason, the nut on top is sometimes called "Jesus nut".

I agree with the previous comments.  Propellers don't fly off.  Therefore the materials used are more than strong enough.  What are you asking for?

Answer is good. If its bolts and nuts and they are over engineered then I can understand. Even though there still would be a component of human error.

Nuts. Technically nuts and bolts.
If you look at pictures of propeller hubs (google "propeller hub") you'll see that the individual prop blades are held onto the prop hub by nuts and bolts holding the hub halves together. Or for wooden props they're bolted directly onto the hub.
If you then take the hub apart, you'll see that the hub is attached to the shaft using nuts and bolts.
I sometimes see people asking why aircraft engineers specify very expensive nuts and bolts for certain parts of the aircraft when a cheaper part can handle an equivalent load. Your question is exactly the reason - it's better to over-engineer certain parts like the prop assembly than have the prop accidentally fly off during flight.

Wouldn't the nuts and bolts also be safety-wired together to prevent them from accidentally unscrewing from vibration?

I think you will find that the nuts and bolts will be threaded in the reverse manner to the force imparted on their.  Hence they will be forced tighter if there is any play.


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Do pilots choose which aircraft to get rated for or is it the employer that decides it?
How do pilots end up flying a specific plane type? Do they choose what type of plane they want to fly and persevere the goal or do you generally get an ATPL and then get a rating based on what your employers aircraft models are?
Example:
I love turboprops and in particular the ATR42, I get employed, if I'm lucky or good, by an airline that has ATR42/72 mixed with other aircraft in the fleet. Do I have a choice as a pilot or do I have to strictly adhere to the aircraft that my employer assigns me?

It depends. In the US and at airlines, seniority rules what aircraft, and what base you start at. Usually, there will be bases for pilots, that are then broken down by aircraft(though not all bases will have options for all aircraft types) and then Captain and FO spots for each group. For example at Delta, at one point, the new hires were going primarily 767 FO spots, at JFK. As their seniority in the airline increases, they can then try to bid to a different aircraft or a captain spot on any aircraft that their seniority can hold, or to a different base. Some may spend a bulk of their career as FOs and just like being a senior FO at a base or type, others will jump around slowly moving up and progressing through various options, depending on the individual.
In the US, and airlines, generally you don't pay for your training for the type ratings. Though, Southwest has in the past required applicants to have a 737, and a few others require the type rating already in hand.

Out of curiosity do you have any means or knowledge to expand the question to include carriers out of the US?

From my understanding, Europe is similar to the US in it. (Though for Europe, it's usually the short haul stuff that is the most senior, versus the US, where it's long haul). The rest of the world(esp where they hire foreign crews), I suspect it's similar, but I have no information on how they operate.

Commercially you can choose most of the time within some reason of what aircraft type you would like to fly, however if you join the RAF for example you won't get an option what to fly, when you join as a pilot you get selected by them for whatever aircraft you will be flying, this can range from small light aircraft, to large transportation jets and even helicopters!
Hope this helps :P

Interesting but are you sure that you really are able to choose?

If choice would be available what happens if all of the pilots select the same type of aircraft?

Yeah, well most UK airlines will let you have a preference. :D


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How do I keep a C172 straight on the centerline during landing/rollout?
During rollout upon touching down, I'd often find myself veering off the runway centreline to the left. Every time I input rudder, I have this feeling that I am getting pulled out of the turn (while that is normal just like turning round a corner at high speeds in a car), and that the aircraft might just flip over. But you see I know my limitations of the car, and that it won't flip unless I yank the steering wheel.
My instructors tell me to just simply keep my feet active, but it's the fear of too much input that will cause me to lose directional control of my aircraft.
Upon touching with my main wheels, to maintain centreline, should I just really apply rudder without fear, even though I feel, and visually believe, that I am going to flip over?
What correct techniques should one apply to keep the aircraft straight during the rollout?

Is there always a right crosswind when this happens, or are you veering left even when there's a left or no crosswind? Or do you veer right in a left crosswind?

Just to calm your fear, more lost control incidents happen as a consequence of too liitle rudder work than too much rudder work.

You might benefit from practicing taxiing along a line, starting out slow to get a feel for how the plane responds to the rudder, and later moving to higher speeds as you build confidence.

My guess here would be that you are not truly tracking the centerline of the runway on final or on roundout.  There’s an easy way to solve that when flying final:  if the runway centerline appears pure vertical from your perspective in the airplane while stationary in your field of view, you are tracking the centerline.  If it has a slight angle to it or is translating, then you’re not totally on centerline.
The differences can be subtle, but can lead to flying a serpentine track toward the runway on final, during roundout, and during ground roll.  It can also result in a touchdown with a slight drift to it which indicates poor piloting technique.
To counter this, first, remember to get the final approach stabilized ie on centerline, on glidepath, and on airspeed as soon as possible after rolling out from the base to final turn.  Remember the more corrections you need to make to do this closer to the runway threshold, the more difficult the approach becomes and the more likely errors in technique will grow unchecked.  Once on a stable final approach, observe the runway centerline and judge if it is a vertical, straight line.  If so, you’re tracking straight down the centerline.  If not, use shallow, coordinated turns to align the airplane so.  This technique will also work when flying a crabbed final in crosswind conditions.
As you roundout, remember you are in slow flight at an increased AoA, so the propeller will tend to pull the airplane’s nose to the left.  Use additional right rudder to prevent this and track straight.
After touchdown, use quick precise feetwork for rudder inputs - think “happy feet happy feet happy feet” - to track centerline.  Also, if doing a crosswind landing, DON’T RELAX AILERON PRESSURE AFTER TOUCHDOWN or the airplane will tend to veer off centerline rapidly.  Remember, fly the airplane all the way through the landing, including the complete rollout.

The chance of flipping due to too much rudder is a very real concern, and perfectly valid.   Flipping can happen quite suddenly due to crosswinds, so you are right to be cautious.
My advice:  On landing, make sure you hold full back elevator.  Many students have a tendency to think they've landed the plane, and then let go of the aerodynamic surfaces like the elevator.  Pilots have a saying, "Fly the plane all the way to the gate!"  Never treat the plane like a car:  It is always "flying", even on the ground.
Holding the elevator full-back after landing takes a lot of pressure off the nose wheel, which makes steering easier and the plane more stable.  It also provides a lot of aerodynamic breaking (you have a large metal board sticking up into the wind) which slows the plane down a lot, in a stable, safe manner.
Also, don't necessarily try to stick to the centerline like glue.  If you try too hard, you may make drastic and sudden rudder changes which can flip the plane.  Instead, work on constantly adjusting towards the centerline.   If you're off too the left, use right rudder to aim back towards the centerline; not to hit the centerline immediately, but to adjust your course towards it over the next 2 seconds.  Then re-evaluate, and adjust your course closer to the centerline again, never actually getting 100% on the line, but always moving closer and closer to it.

Great advice abelenky, thank you so much for filling in the knowledge void! Gonna try it out the next dual check and have my instructor reassess my skills.

@ymb1:  The nose gear won't be off the ground, it will just have less pressure/force on it.  The oleo strut won't be compressed, and the wheel will have an easier time turning side-to-side smoothly.

Lots of good info in the other answers, but one thing you need to keep in mind when landing is that the nosewheel on Cessnas drops down and aligns with the fuselage when you are in the air.
As a thought experiment, think about what the nosewheel will do if you take your feet off the rudder pedals just before touchdown and the airplane is lined up with the centerline. It should go straight ahead and very minor input will be needed from the rudder pedals.
Now think about what will happen if the nose is not pointed straight down the runway and the rudder pedals are neutral. Now the plane wants to head off to one side. The farther off your nose is from the centerline the more rudder input will be needed.
Finally, think about what would happen if you were correcting for a crosswind but didn’t bring the pedals back to a neutral position before touchdown. The nosewheel will touch down aligned with the aircraft and then immediately turn in the direction of the down pedal. You’ll probably overcorrect for the sudden change in direction, then overcorrect again in the opposite direction.

"Trikes" can flip forward left or right, which is why many people prefer "quads" (or having the single wheel in the rear). However, with 2 main wheels down only rudder input will still swing the nose left or right with little danger of flipping.
When you are at landing speed and after mains touch down, the rudder is your friend, use it.
Very good advice to hold nose gear off while your are slowing down.  If you are slightly off center, continue controlling the aircraft as it slows down (this includes aileron inputs too) and keep "flying" the plane until it rolls to a complete stop.
But abrupt inputs or sharp swerves are not needed.  Easy does it, and differential braking is also helpful.  As you gain experience, you will feel more comfortable with its limits.
I would not rule out a taxiing lesson to test and refine your ground control techniques.

Differential braking during rollout sounds scary to me, and might induce a sharp swerve. During the roll along the runway, wouldn’t it be better to apply equal braking pressure until you’re ready to turn off into the taxiway?

Absolutely, as a matter of fact I always made "heels on the floor" part of my check list before taking off.  Differential braking is useful once you are down to around 5 mph to help turn.  But hey, if the rudder starts to flip you over, then opposite rudder!  (Rudder away from trouble). But one writer here really hit on a good solution, think centerline on your final, the sooner you line up with it the easier it will be.  You can always go around if you are off.


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Why isn't there a single best airfoil for subsonic flight?
Why does airfoil used by modern subsonic aircraft (this is true for transonic airliners too) vary from plane to plane?
Haven't we figured out a single best airfoil shape, with the highest lift to drag ratio?

For the same reason why we do not have one single type of aircraft flying all commercial and military missions worldwide: flight has many variables, and there is no single optimum solution.
First off, you mention transonic flight in your question, which usually involves sonic flow at over at least part of the airfoil, which significantly alters the desired properties of the airfoil. To follow with this example, have a look at supercritical airfoils. They exhibit good qualities for their typical flight regime, albeit at substantial development and manufacture cost. A simpler NACA airfoil would be more cost-effective for applications that do not require high-subsonic cruise efficiency as one of their main points.
Additionally, there are more specific situations, like flying wings, that usually use reflex camber to deal with the lack of horizontal stabilizer. Again, this modification is rendered into pointless weight and cost if used outside this particular scenario.
Now, to specifically address your Lift-to-Drag requirement: that is simply not a good way to decide "the best airfoil for subsonic flight". High efficiency is important for aircraft with a mission that emphasizes endurance. This category is populated by gliders, and is indeed full of highly optimized designs ($C_L / C_d \approx 50$ is not unheard of) with little differences, mostly due to structural considerations.
Now take another subsonic aircraft with an emphasis on endurance: the maritime patrol aircraft, the best known example here is probably the P-3 Orion. A glider airfoil would be terrible because of the need for thickness to accommodate a spar that can carry the engines without breaking under a bad gust. Instead it uses a NACA 0014-1.10 at the root, transitioning to a NACA 0012-1.10 at the tip, meaning the designers had to compromise between two airfoils even within the same wing.

In fairness, OP did specify "highest lift to drag ratio" and "subsonic flight".

@aCVn fair enough, that did not register with my brain for some reason. Maybe something to do with the fact that next sentence about transonic flight violates the subsonic condition from my point of view. I'll amend the answer.

Glider airfoils come in a variety of thicknesses, up to 19.6% for the root airfoil on the ASK-21. There surely is a glider airfoil which would improve the P-3 considerably, without the wings braking off.

Consider that the Orion has a top speed in excess of 400 kts,  cruise speed of about 330 kts (maybe less while loitering, but Google won't say :-(), and a stall speed of 97 kts.  A typical glider might thermal at around 50 kts.  Why would you expect the same airfoil to be optimum for both?  Likewise, if you're designing say a bush plane, you want low stall & cruise speed, while passenger jets have high cruise speeds and long, paved runways.

@jamesqf: A well-chosen glider airfoil will not be optimum, but heads and shoulders above the NACA 0012, even if it is modified in case of the P-3.

In aircraft, size does matter.
Smaller aircraft flying at the same speed, air temperature and altitude than a larger aircraft have a smaller Reynolds number which characterises the boundary layer flow. A smaller Reynolds number allows for more laminar flow but demands a less steep pressure rise in order to avoid early separation. So there is a different airfoil for every design Reynolds number, even at the same lift coefficient. Very fast subsonic designs have to deal with pockets of supersonic flow which put very different demands on the airfoil's shape.
Next, airplanes come with a big variety of wing loadings, from 40 kg/m² in gliders up to the nearly 1200 kg/m² of the Rockwell B-1B. This translates into a very different range of speeds, so the variation in Reynolds numbers is much wider than the size alone implies. Heavier aircraft tend to have higher wing loadings and need to add high-lift devices, which put their own demands on airfoil shape.
Then consider aspect ratio: Depending on the purpose, the optimum aspect ratio of subsonic airplanes is anywhere between 4.5 and 50. The smaller value is for flying fast and high g loads, like in aerobatics, and the higher value is typical for high performance gliders. The short and stubby wing will need a different airfoil than the long and sleek one.
Also, ideally you should vary the airfoil within one wing, depending where along the span you look. The root will benefit from a thicker airfoil than the outer wing, where a wide angle of attack range from aileron deflections and roll speeds needs to be tolerated.
And then there are designs wich care less about L/D but need to have ideal stalling characteristics and no camber: Aerobatic aircraft have quite different airfoils from all the others, and with good reason.
This only scratched the surface of the issue, but I hope I could get the general idea across: Even when optimum L/D is the design goal, the best airfoil shape differs with the other parameters of the aircraft.

"The root will benefit from a thicker airfoil than the outer wing, where a wide angle of attack range from aileron deflections and roll speeds needs to be tolerated." - Are you saying that the outboard part of the wing will use a thinner airfoil in order to tolerate a wider range of angles of attack? I thought that thicker wings worked at a wider range of angles of attack, not thinner wings.

@TannerSwett: Yes, indeed, although it sounds counter-intuitive. A thicker airfoil has higher suction on both surfaces from the displacement effect, and that will add to the recompression gradient in the rear part of the airfoil. In effect, a thicker airfoil will tolerate a lower maximum angle of attack. Please don't confuse this with the leading edge radius: Here, a thicker value will indeed help to tolerate a wider angle of attack range because the suction peak near the nose will be lower.

Airfoil performance will depend on air speed and density. A Cessna 172 and a DeHavilland Dash 8 are both operating at subsonic speeds, but in very different speed and density regimes. Jet aircraft and propeller aircraft mostly operate in different speed regimes as well. The details of how the wing is laid out will also affect performance. The chord length will change the Reynolds number that the airfoil experiences. The wingspan and sweep angle will also affect the performance of the wing. Different wing geometry in different flow conditions will drive different choices in airfoil.
All of these differences are because different aircraft are designed for different kinds of flying. Gliders, for instance, are designed for a very high lift to drag ratio for minimal drag. Bush planes will place a high emphasis on low speed performance to reduce takeoff and landing distance. Passenger aircraft are willing to sacrifice some of the aerodynamic efficiency to fly faster to reduce trip times. Structural considerations as well as infrastructure will limit wingspan and influence thickness.
Airfoils will also have different characteristics in situations such as stall. For most aircraft, you don't want the stall to be sudden and severe. But for aircraft designed for performance, such as aerobatics or racing, they are willing to sacrifice this for lower drag.
You will see the same trends in nature, as there are many different types of wings. Small wings are very different from large ones. Wings designed for soaring will be different from wings designed for speed.

Generally, we know the best airfoil shape, but we do not know the best speed.
Lift is proportional to the SQUARE of speed.  The Cessna 172 is barely above stall at 50 knots, but putting in large amounts of down trim at 100 knots.
But the airliners have it figured out.  They can fly in a huge speed envelope of around 150 knots to 550 knots by changing wing configuration, more specifically changing camber.
The highest lift wings are heavily cambered with a concave underside.  These "thin undercambered wings" were popular in the early days of flight, but fell out of favor as speeds increased.  A thin wing with less camber produces adequate lift at higher speeds but with MUCH LESS DRAG.
So the airliner reconfigures to a thinner less cambered wing for high speed cruise, but returns to the early days to greatly reduce landing speeds.  Perhaps we will see a Fiesler Storch with retractable slats.


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What kind of airplane is this that has blue cheatlines and a protruding side cover?
Consider:

Which airplane is this? I suspect it is Russian, but it's too poor quality for me to tell. I'm pretty sure it is military.

got challenged by a friend so its pretty vauge, I suspect the white bulb is some sort of signal intelligence thing or a radar of some sorts, the blue lines are pretty common on russian military aircrafts for example the an-72 wich makes me think it is russian.

After looking at the accepted answer, I look again at the blurry OP photo and see Ил-18, but I never would have been able to read it if I hadn't known what to look for!

I'm voting to close this question as off-topic because it is a puzzle, not a question about aviation.

@DavidRicherby I'm not sure why the reason for asking a question matters, if the question itself is on-topic? The OP is asking for aircraft identification based on limited information, and that's definitely on-topic here.

@Pondlife Because it serves no purpose other than "just mindless social fun", something that should be avoided because they have no lasting value and does not even help OP.

@Pondlife The question is just a game. The only reason the question exists is because somebody who knew the answer deliberately obscured that answer and requested that it be found again. One could create an infinite number of such questions, none of which serve any purpose other than being questions.

Below is the full photo, it's an Ilyushin Il-22PP Porubshchik electronic warfare plane.

The Il-22PP Porubschik was designed in late 2000s on the basis of the Soviet Il-18 passenger aircraft made in mid 1970s. According to media reports, the aircraft is capable to disable most modern electronic warfare systems installed on foreign planes, air defense systems, as well as unmanned aerial vehicles.

Image and article: https://www.ruaviation.com/news/2017/3/31/8382/


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Why don't airports use different radio frequencies/channels for each plane to prevent communications from interfering with each other?
There have been serious safety incidents where multiple airplanes using the same frequency caused simultaneous messages to garble each other. For example, at Pearson International Airport (2020-03-07), two planes nearly collided on the runway, because a small Embraer 190's message that it was rejecting takeoff was drowned out by a routine message from a Boeing 777's more powerful transponder. Something similar happened during the 1977 Tenerife disaster, though it likely would not have prevented the incident.
I once worked a job where people communicated by walkie-talkie frequently. Every department had its own channel, so that it could focus on its own most relevant communications. Why don't airports, which are much more safety-critical than my job was, segment communications like that?

Possibly some day when comms are fully digitized at both ends, say by a system that replaces VHF transmitters/receivers with a digital satellite network, that this can be achieved with software where pilots hear what other pilots and ATC are saying to each other, but are prevented from stepping on each other. Give it 20 yrs.

So you lost situation awareness, e.g. other pilots clear to land in your runways (and this extra listening prevented many incidents). It would not be complex to airports to know about talking over (just put antenna and receivers around the airport, and maybe directional: inside/outside perimeter, software can easily see if two aircraft are talking at same time, and plus: ATC can have exact position of who is talking. No need to change infrastructure, just to add few new unconnected systems (and relatively cheap)

There is a better approach to solve that issue: https://aviation.stackexchange.com/questions/102332/why-are-we-still-using-audio-to-communicate-between-atc-and-pilot

This makes little sense, the entire point is everyone is supposed to hear everything.

Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed.

Note that while there is no per-aircraft segmentation, there is segmentation, with many different frequencies for different operations. See for instance https://www.iflightplanner.com/Airports/KJFK (there are a few duplicates, but you can still see there are quite a few).

@Fattie that's a very succinct answer, but an answer nonetheless

lol thanks @ChrisH . You know, there's a fascinating problem on the few "expert" sites, an example is the physics one… ... ... ... Someone will ask a very basic question for example "Does the Earth go around the Sun, in a circle of square?." The answer to that would be "Circle." But, because the site is packed with basically the world's leading research physicists, someone, indeed everyone, will launch into a spectacularly explanation about the effects of the latest competing quantum gravity theories on the nature spacetime and geometry. But the actual answer was "Circle."  !!

I think there's confusion about "a small ... message ... was drowned out by a ... more powerful transponder" - AIUI, any two people talking on the frequency at the same time will cause either both messages to be garbled or only one heard. I don't think the category of aircraft or "power" or transmitter matters (beyond a higher wattage will broadcast further, but I'm quite sure that's heavily regulated, too).

@Fattie Do you really think this question is that simple, so simple like the Earth's orbit as to be stupid?? Its popularity certainly speaks otherwise. I saw that the current practice had caused safety incidents. I knew that it's taboo for pilots to communicate with each other, so the current practice didn't seem at all necessary. And I knew that there was a solution. So I think it was a fair question and I'm insulted that you would think I'm a fool who deserves a simple one-line answer.

?  I have no idea what you mean, it's a great question, indeed I voted it up. You rock.  Comments on the internet sometimes seem "abrupt" because there isn't always time to type "filler" you know. (Smileys and such!)  @PurpleP

There are different aspects to assess:

Does that solve problems?

Is it technically feasible and economically acceptable?

Does that create other problems?

Problem solved
It can solve issues like the one you described.
However in that case, the possibility of misinterpretation of ATC directives is already managed by procedures: Important directives must be acknowledged explicitly (read-back) so that both parties can check they have a common understanding. So the added-value is limited, and if an effort should be done, it would likely be first on the proper use of procedures (which is a key factor in Tenerife accident).
Technical and economical feasibility
At the time the communication process was standardized by ICAO, only analog modulations, mostly AM and FM, were really usable. Selective calling (selcal) was used, but only on HF, not on VHF, and selcal doesn't protect against interferences anyway, as the channel is still unique.
AM/FM required different carrier frequencies to work with different channels, meaning a large spectrum occupancy for each airport / ATC unit.
Because the spectrum is limited, it must be shared between airports, different airports will use the same channel, creating interferences between airports (even distant from 200+ km). This issue is likely worse than same unit interferences: Aircraft would listen to different ground stations on the same channel, but each ATC unit would not be aware of the other units.
So technically speaking individual channels were not really feasible.
New problem created
While multiple channels would help solving problems that are supposed to be already solved by procedures, it also creates new ones: Loss of general / common awareness. We could likely find accidents that were avoided by listening to a common frequency.
--> @DeltaLima found already one (thanks). Watch the video and see how a common channel and a professional crew, aware of the overall confusion at the airport, just saved lives.
It's really difficult to say if there is a mean benefit in using individual channels, this requires a serious analysis nobody can do here.
How the problem could be solved today
Today there are digital modulations and spread spectrum technologies (frequency hoping) allowing sharing the same spectrum slice for many channels, resisting to interferences. This mode of access to the channel is used since a few decades by Bluetooth, WLANs, cellular networks, GNSS, etc. There is no doubt the next generation of ATC, currently developed by FAA and Eurcontrol (SESAR) will use spread spectrum communications.

https://youtu.be/equVF3ULVw8?si=tO-fs34OK49YHrGK for example. Would have been deadly with separate channels

@DeltaLima. Thanks for the suggestion, and the link (added to my post). Impressive case of a professional crew saving lives.

And as a negative example, the runway incursion this morning at DCA (tower cleared an aircraft to depart the same runway that ground cleared another aircraft to cross) was a case where having the two crews on different frequencies worsened the situation, as it meant neither was aware of the conflicting clearances and couldn't hear each other.

If you're talking about modern solutions, antenna arrays are also relevant. Modern radio can be directed pretty accurately, which would greatly decrease interference between airports.

Because it is actually critically important that pilots all hear what is going on around them.
In a perfect world, ATC could act as the go-between for everything. But in reality there are times when an airport gets very busy and every second counts. If plane 1 hears that plane 2 is having trouble getting off the runway or that plane 3 is rejecting takeoff, or whatever, that means plane 1 now knows about the problem before ATC contacts them directly.
In some cases, a pilot may notice a problem such as debris on a runway that affects others and by using a common frequency the other planes are notified immediately instead of waiting for ATC.
I am sure there are plenty of specific examples that the professionals can provide.

I like your answer, but you may want to consider that all these aircraft on "discrete" frequencies are still talking to one controller. If no aircraft can hear other aircraft, there's no reason to not key up and communicate with ATC. Meanwhile, the poor controller has four aircraft talking at the same time, all going into one headset. I controlled aircraft at a very busy facility for many years, and simultaneous transmissions occur, but with the present system and pilot etiquette, it doesn't happen a lot. Additionally, CPDLC will reduce frequency saturation to reduce the problem.

I remember once landing at an airfield with parallel runways.  I crossed final for the right runway and looked over to see the airplane cleared to land on the right runway lined up with the parallel taxiway to the right.  I alerted ATC about the situation, but in all actuality, I was letting the other pilot know they were lined up with the taxiway.  The plane banked left to line up with the runway before they landed without incident.  I can only imagine that they were quite thankful for the observation on my part.

that would better solved by an ai acting as communicating device at the relay, so it can verify what is important and what is not. Remember humans are TERRIBLE at filtering information, how much you train them they will focus on a problem one at a time. AI/computers can much better keep overview; so question is actually: why is ai not used as filter. The chance for that to break is way less than the chance for a human to have information overload.

@paul23, it's going to be a very, very long time before AI is used in such a safety critical and risk averse role as ATC (if it ever happens).

@Crazymoomin why? That's just like denying safety because of politics? Why are people so arrogant to consider themselves better than a computer? Is there some kind of proof regarding that?

@paul23 Because if the AI makes a mistake and there's burning wreckage on the ground, that's the end of that. It has to be pretty much perfect out of the gate, and no AI currently available is anywhere near infallible enough.

@paul23 AI can't keep a better overview, just a faster one. Your average AI system beats an untrained human, but a trained human beats any AI system we have yet invented (at ATC, not chess). Worse, an AI system's comprehension would be alien to the humans using it as mediator, so pilots wouldn't be able to easily predict how it'd respond to their communication attempts. People are investigating the use of AI in other areas, but not for handling comms.

@wizzwizz4 well the weakest link is the pilot, we need remove the whole idea of human communication in the end and need to use a more strict protocol.

@paul23 The weakest link is the black swan. The second-weakest link is the weather. Pilots are pretty good at mitigating both of those, and computer systems are not. How does one communicate "someone's flying a load of drones with lights to make it look like the airport is half a mile to the south" via your strict communications protocol?

ATC, not chess - that's key. Chess has many possibilities but all clearly defined. The real world always comes up with surprises.

There's an excellent example of this on the VASA YouTube channel. ATC lines up a departing plane behind the landing. Departing plane calls clearing the runway but is vague and unconfident about its position. ATC clears plane for takeoff, but the pilot says "we're not taking off until we have more clarity about the other plane's position". It turned out the landing plane had strayed onto a taxiway that would have taken it across the runway. They would probably have noticed, but "probably" is not the standard we are looking for here.

Having separate frequencies all being handled by the same controller isn't going to solve two people trying to talk to the controller at the same time, just the opposite. There's rhythm to the communications on a common frequency,  and you generally know when to press the transmit button, even on a busy frequency. For example, I may want to talk to ATC, but they are saying 'G-NWFT turn left to 150, climb and maintain 5000'. I know that FT is going to respond shortly with something like 'left to 150, climb and maintain 5000, G-NWFT' and I will wait for them to finish before I transmit. If everyone's on a separate frequency you'll have frequent instances where several will be talking to a single person at once.
A common frequency sounds like a mess to a layman, but to a pilot it's a wealth of information that gives situational awareness. Listening in will give me an idea of how congested the airspace around me is, and where other aircraft are in the vicinity as well as where they are going. I'll get an idea of aircraft types, and by listening to the pilots I may glean other useful information like their workloads and capabilities. For instance there may be a student pilot, someone with a technical issue, or I may get bad vibes and want to avoid someone.
It's also important for safety. ATC isn't perfect, a common frequency gives the opportunity for pilots to detect mistakes and take action. For instance, if I'm on short final approach and have been cleared to land on runway 18 and I hear 'G-NYLY line up on runway 18, cleared for takeoff' I know that ATC have just made a very dangerous error and told an airplane on the ground to depart on the runway I'm about to land on. I can then go around. Or, if I am the pilot of that airplane on the ground that's just been cleared onto runway 18 and I've been listening on the common frequency I would realize the danger and refuse the clearance.
Yes, occasionally two people will talk on the frequency at the same time, but it's rare and although it's inconvenient when it happens it's worth it to hear everything else.

"A common frequency sounds like a mess to a layman" is certainly true, but it's pretty easy to get a handle on that mess.  The rhythm you refer to helps enormously, as does reading back to give the layman a 2nd chance to pick up the message.  Years ago I worked for the UK's NATS  timing airport movements for efficiency monitoring, and within a couple of hours of practice (with a good view from half way up the tower, and a few paper reference documents) everything made sense

How is this handled for on-route controllers? Are the airplanes high enough in the sky that even at opposite ends of the controllers area of control, they can still hear each other. I know in the maritime world with coastguards this can actually cause non-trivial problems, because the coastguards are often using multiple receiver stations where ships can't hear whats going on at other receiver stations.

As l long as there is line of sight you can hear airplanes for pretty long distances @user1937198. I'm a GA pilot in the UK, and you get some frequencies which are used over very large areas, like London Information, but that's an information frequency for airplanes operating outside of control areas. There are times I'll know someone is transmitting from a long distance but not be able to hear them, but if they're that far away it doesn't impact me. If there's an airplane that ATC can't hear because of terrain or interference another airplane may relay the message, that's pretty rare.

Not having a common frequency would be a challenge to handle the logistics of managing the frequencies needed, especially in crowded airports.  If each plane had a different frequency, they'd need to get new information on which specific frequency to use each time they visit an airport.  With a common known frequency, an incoming pilot would know that each time they were on a flight that needed to contact a specific airport tower, they'd already know the frequency as it would be well known and non-changing.  Predictability is important, which is why there are standards for communications.


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What are the differences between third-party base maintenance, line maintenance and component maintainance?
These are related to MROs(Maintenance, repair and operations) activities ?

These are indeed related to MRO activities.
Line maintenance generally refers to minor, unscheduled or scheduled maintenance carried out on aircraft that occurs at or near the gate or terminal (Tarmac), launch area, ready area, hardstand or alert area. This is usually carried out by the operator. This is the basic level of maintenance and is limited by the ground support equipment available.
Third Party Base Maintenance refers to the base maintenance (i.e. maintenance done in aircraft hangar with special tools) carried out by a third party- other than the manufacturer and the operator. This is usually the next level of maintenance.
Component maintenance refers to the maintenance carried out on some component removed from the aircraft (for e.g. rotable or LRU) due to fault or completion of specified service time in a dedicated shop or lab. These are usually done for avionics units, engines, transmission items etc. This is usually done by the OEM or their authorized agent.

Base maintenance: The maintenance work is done in a hangar, with special tools and equipment. Third-party just means that someone else than the operator of the aircraft is running the shop.
Line maintenance: The work is done on the flight line, basically on the apron out in the open.
Component maintenance: The work is done on some component that had been removed from the aircraft before. Typically, this item had been swapped against a good one in line maintenance and is now looked at in detail in a specialist shop. Items like this are called LRU (line-replaceable unit). Components could also be as large as a whole engine which needs to be serviced in a special shop and is removed during base maintenance.


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What avionics systems are interfaced with ACARS?
I am a librarian looking for sources of information to help answer the following questions for one of our researchers: 1. what avionics systems are interfaced with ACARS and 2. how is the interaction between ACARS and these systems designed to occur in practice? As I am not very familiar with this subject, any information or pointers to information would be appreciated.

Hi, Welcome to Aviation.SE! It would probably be helpful for getting more precise answers if you could specify more precisely what aircraft you're interested in.

ACARS, otherwise known as Aircraft Communications and Addressing Reporting System, is almost always interfaced with the #3 VHF Communication System. Ordinarily, that #3 VHF system is set for "Data Mode." Some aircraft have the ability to use the #3 VHF as an auxiliary communications system by putting it in voice mode, and other aircraft dedicate it to strictly data mode.
However, I have seen operators tie the ACARS system to the #2 VHF system when there is no #3 VHF system installed. In this case, it's ordinarily up to the pilots to switch between voice and data mode on the #2 VHF system. I can only imagine how annoying that must be!
While VHF is most frequently used on domestic flights, its relatively short range does little good on international flights over water. On aircraft meant for such long hauls, the ACARS system is usually tied to the HF radios and the SATCOM system. If I recall correctly, the ACARS system opts for VHF communications as the primary method. If there is no VHF available, or its rendered inoperative, the secondary method is often SATCOM; failing that, the HF systems.
In order for the aircrew to interface with the ACARS system, there is some sort of display with a keypad they interface with. On the vast majority of aircraft I've worked on, that display is often referred to as either the MCDU or CDU. On aircraft with MCDU's (Multifunction Control Display Unit), the pilot can select the mode they prefer to be in (ACARS, FMS, etc...), and can interface with the appropriate system through that singular central display - kind of like selecting what program you'd like to run on your computer. If the pilot selects ACARS, they can interface with all the relevant data pertinent to the ACARS system. If they select FMS, they can interface with the FMS (Flight Management System); and so on and so forth.
On aircraft equipped with CDU's (Control Display Units), usually there are completely separate displays/interfaces for the FMS and ACARS. In that case, there are a plethora of options from a number of different manufacturers. I've seen touch screens, a secondary but different style CDU dedicated to ACARS communications, and so on and so forth. It's wide and varied.
The MCDU and/or CDU usually interfaces with an ACARS Management Unit. That management unit is ordinarily the "brain" of the ACARS system. All pilot interactions with the MCDU/CDU are sent to the ACARS system, usually via a 429 Databus. The best way to think of this is that the MCDU/CDU setup is like your computer monitor and keyboard. You input data and see those inputs with keyboard and monitor, but the real "action" is taking place with the memory and processor "behind the scenes." That behind the scenes action is the ACARS Management Unit.
The ACARS Management Unit (MU) can then interface with a variety of other systems and subsystems largely based on operator configuration. Some operators choose to log crew hours with the use of OOOI (Out, Off, On, In) times based on subsystem inputs to the ACARS MU. For example, when the beacon turns on, an input to the ACARS MU generates the "Out" time. When the aircraft takes off, the ground sensing mechanism of the aircraft sends a separate signal to the MU letting it know that the aircraft has left the ground. When that ground sensing mechanism changes state again (aircraft lands), it sends another signal to the MU letting it know that the aircraft has landed. Finally, when one of the doors (usually the L1 door) is opened after flight, the ACARS receives a signal from the door switch, and the "In" time is generated.
Sometimes the MU is tied to an ACARS Aircraft Personality Module (APM). This is usually just a separate unit with DIP switches set to a certain configuration so no matter what MU is installed on a particular aircraft, it can pick up the "correct identity" and act accordingly. Other times, there is no APM and the configuration control is handled exclusively by the software load often verified by the mechanic after installation.
Also, with the advent and implementation of ADS-B aircraft, GPS position data has begun to tie in to the ACARS system. If you want to see a mechanic roll his eyes, ask him to do a full functional check of the ADS-B system in the rain or cold. It's hours long, it never fails, and the consequences of what happens if it is failed are negligible. Most of us see it as a giant waste of time... But I digress.
Finally, I'd like to note that all of this information is with the caveat that every operator is different, and so is every aircraft. There are endless combinations of systems that could or could not be tied into the ACARS system. Generally speaking, though, what I've mentioned above is a fairly standard installation.

I'm Europe ACARS is typically used over VDL mode 2 or a satellite link like Inmarsat Classic Aero. An example of an application which is used over ACARS is CPDLC.


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Why do supersonic planes fly higher than subsonic airliners?
I saw on wiki that a boeing 737 flys at about 38,000ft and a Concorde flys at 60,000ft. What made it possible, and advantageous, for Concorde to fly so much higher?

As titled, this question is not a duplicate. It asks why. The question body needs help though.

@digitgopher the one marked as duplicate contains the answer to the why as well. This one is then correctly closed.

It was necessary for Concorde to fly much higher, and not a voluntary decision. A higher cruise altitude increases the demands on the airframe (examples: Combustor size, pressurization) and is only accepted when it comes with sufficient benefits.
An airplane needs a certain wing area for realistic take-off and landing distances. Also, details like the maximum tire speed need to be observed, so the wing area must be large enough to create enough lift at low speed.
Lift is proportional to the square of airspeed and air density. The faster a plane flies, the lower the density can be to create sufficient lift. By flying more than twice as fast as a Boeing 737, the Concorde could fly at a fourth of the density by this reasoning. Air density (symbolized by the Greek letter $\rho$) halves approximately with every 5500 m of altitude increase.
However, the slender wing needed for low supersonic drag is not as efficient as the sub- / transsonic wing of a 737, so a little more density was needed for the Concorde. The 737 is designed for cruise between 8000 m ($\rho$ = 0.526 kg/m³) and 12000 m ($\rho$ = 0.312 kg/m³), and the Concorde flew between 16500 m ($\rho$ = 0.154 kg/m³) and 18000 m ($\rho$ = 0.121 kg/m³).

While perfectly valid, can you explain how your first paragraph relates to the question? (Note: I didn't get much sleep last night, so I may have simply missed something.)

@FreeMan: Concorde could fly at the same altitude than the 737 it its wings were smaller. Then it could afford to have smaller combustors and a much smaller wing. Flying higher is creating a lot of problems, and was the result of the rather large wing, which in turn was required to allow Concorde to operate from existing runways.

So, A) the Concorde needed a slender wing for good supersonic performance, which B) doesn't produce as much lift at lower speeds, requiring C) a larger wing for realistic take-off/landing speeds, which lead to D) a higher cruise altitude because of E) the extra lift available at higher altitudes from the very large wing?

@FreeMan: Yes. Airplane design can be so easy!

Makes you wonder why people go to school all those years to get some sort of engineering degree. I learned that in a couple of month's reading on a Q&A site!  ;)

The air is much thinner up that high and therefore there is much less drag/resistance, therefore an airplane won't heat up as much as it would going the same speed at a lower altitude, and it requires much less thrust/fuel as well (economics matter too when you have paying passengers).

Aeroynamic heating depends on airspeed squared. Lower density will only reduce the heat flux, but not the temperature. If an equilibrium is reached (pretty likely for an airliner which cruises for several hours), the heat flux is not a factor anymore.